Many years ago Dr. Shannon proved that it was possible to encode digital data and effect improved data transmission. This theory was known as the channel capacity theorem. His theory only proved there existed coding techniques but did not consider the means to achieve a net coding gain. One class of codes were called block codes and a separate distinctly different class was referred to as convolutional codes.
It became apparent that convolutional codes were in general superior to block codes because soft decision techniques were used in the convolutional decoders. The technique which achieved general acceptance is the Viterbi decoder well described in texts and other publications. Most convolutional decoders are of this type and provide excellent performance. This type of decoding is described under U.S. Pat. No. 3,789,360 for Convolutional Decoder.
The technique of decoding short block codes was significantly advanced with the development of a Multiple Decoding System, U.S. Pat. No. 4,038,636 by George D. Doland. The performance of the Multiple Decoding System is also excellent but is not particularly suited to data transmitted consisting of 8 bit data words or data in bytes of 8 bits.
In order to achieve higher coding gains which are theoretically possible concatenated systems are now being considered. The Viterbi Decoder as presently implemented does not provide a quality signal output. In current concatenated systems, a hard decision decoder is concatenated with the Viterbi Decoder. This invention consists of two soft decision decoders concatenated and was the result of improvements in both the Viterbi Decoder and the Multiple Decoding System referenced to achieve a concatenated system yielding very substantial improvement in performance not heretofore achievable.